Electrophoretic Deposition (EPD) is a technique which is used to deposit particles from a slurry under an external electric filed. Ceramic, polymer and metallic powders have been successfully deposited using this technique.
Functionally Graded Materials (FGM) are composites in which the materials composition varies with respect to spatial locations. Composites are developed to obtain enhanced materials properties. However, material interfaces generate many problems, especially residual stresses which often obstruct structure/component integrity and reliability. FGM enable smooth transitions between materials. Therefore the residual stresses are minimized.
The traditional methods of producing FGM include slip casting. For example, Basil et. al., U.S. Pat. No. 5,498,383, described fabrication of complex shape geometries by slip casting to produce FGM. However, this method requires the preparation of slurries with different composition ratio. The graded structure was achieved by circulating the slurries. Therefore, the process takes much longer time than the invented hybrid process.
EPD as a method to produce FGM was first explored by Sarkar and Nicholson. Most of the research to produce FGM by EPD are focused on organic solvents, such as ethanol, acetone, IPA, methanol, etc. The reason here to use organic solvents instead of water is due to the bubble generation in aqueous EPD. The bubbles formed by hydrolysis of water will lead to pinholes which severely undermine the quality of the deposits. Lucek, U.S. Pat. No. 4,357,222, described an electrophoretic slip casting process. However, in order to avoid the bubble formation and decomposition of deposit, very low voltage was used in this invention. The low voltage inevitably limits the shaping rate so it's not efficient for batch production. Ryan William et. al., U.S. Pat. No. 4,121,987, developed an electrophoretic slip casting procedure in which an electrically-conductive porous carbonaceous mould is employed. An osmotic pressure is applied inside the mold in order to remove the gas bubbles formed by the electrolysis of water. It is difficult to control this slip composition so it's not convenient to produce FGM. Moreover, the deposition electrode has to be carefully manufactured since the irregularity of the cathode will lead to local defects in the deposit. All the shortcomings prohibit its application especially in producing FGM.
Until 1994, EPD was believed to be a faradic process, in which the reduction of charged particles has to occur at the deposition electrode. All the above-mentioned inventions didn't realize this characteristic and involved complicate processing techniques. In 1994, Sarkar and Nicholson proved by studies that EPD is a non-faradic process. They developed the membrane method to overcome the problem of bubble formation. However, the shape of the deposit is difficult to control. Therefore, the membrane method was limited to simple shapes. Clasen et. Al further developed the membrane approach by using a polymeric porous mold. Again the cost of mold manufacturing prevents its wide application. Moreover, his research did not address the fabrication of FGM using aqueous slurry.
The current similar technology such as slip casting can produce FGM but it is a very slow process so it is not suitable for batch production. Other technique such as powder stacking renders sharp interfaces causing residual stress build-up. In addition, organic solvent-based EPD is not environmentally friendly.